GABAB-RI receptors in serotonergic neurons:: effects of baclofen on 5-HT output in rat brain

The activation of GABA(B) receptors hyperpolarizes 5-HT neurons and reduces cell firing. In situ hybridization showed the presence of the GABA(B)-RI receptor transcript in virtually all 5-HT neurons of the dorsal and median raphe nuclei (DR and MnR, respectively) whereas the GAD transcript was present mainly outside these nuclei. The systemic administration of baclofen increased the in vivo 5-HT release in the DR, MnR and several projection areas. As shown previously in the DR, the application of baclofen in the MnR increased the local 5-HT output. Thus, although 5-HT neurons contain inhibitory GABA(B)-RI receptors, baclofen increased 5-HT release in some brain areas, likely by a preferential action on terminal GABA(B) autoreceptors in inhibitory inputs to 5-HT neurons. The scarcity of GAD-expressing cells in the DR and MnR suggests that these inputs originate mainly outside these nuclei. NeuroReport 11:941-945 (C) 2000 Lippincott Williams & Wilkins.